N2Development of Thin Film Silicon Solar Cells
薄膜シリコン系太陽電池開発
Akihisa Matsuda
松田彰久
Research Institute for Science and Technology, Tokyo University of Science
2641Yamazaki, Noda-shi, Chiba 278-8510, Japan
東京理科大学 総合研究所，〒278-8510 千葉県野田市山崎2641
Recent progress is introduced in the development of thin-film silicon-based solar cell which is one of themost promising candidates for low-cost high-efficiency solar cells in the field of the photovoltaic solar
energy conversion in the near future. Importance of the control of optoelectronic properties in thin film
silicon materials, amorphous silicon and microcrystalline silicon, during film growth on the basis of
microscopic understandings of their growth processes is emphasized.

1. Introduction
Solar energy is highly expected as clean andinfinite energy resources, and photovoltaic solar
energy conversion is recognized as the mostconvenient electricity-generation method among
many technologies proposed to utilize the solar
energy. Therefore, development of low-cost andhigh-efficiency solar cell is crucial issues to realize
the large scale introduction of photovoltaic systemand to cover a mass of electricity consumption inthe future world.
A variety type of thin-film type solar cells suchas hydrogenated amorphous silicon (a-Si:H),hydrogenated microcrystalline silicon (µc-Si:H),
cadmium telluride (CdTe), copper indium
di-selenide (CuInSe2) etc., have been proposed .to
reduce the production cost of solar cells. This issimply because single-crystalline silicon andpoly-crystalline silicon solar cells, the most popularsolar cells at present, take high cost for theirproduction due to two main reasons.
First,
light-absorption coefficient in crystalline silicon isso low due to its indirect-optical-transition
properties that more than several-ten micron in
thickness is required to absorb sufficient sunlight.
Second, the production process of crystalline siliconsolar cells from source gas (trichloro-silane) takes
high-temperature processes more than 1000 C atleast three times and also involves slow process(crystal growth from the melt).
2. Thin film silicon solar cell
Thinking of the environmental problem andnatural abundance of constituent of solar cells,
silicon-based solar cells are considered as the mostpromising candidate among many candidates
mentioned above.
From this point of view,hydrogenated amorphous silicon (a-Si:H) has

attracted an attention as suitable material for solar
cells, since it shows no indirect-optical-transition
properties and this material is popularly prepared byso-called low-temperature process of 200 C using
plasma enhanced chemical vapor deposition(PECVD) method.
However, a-Si:H solar cell shows photo-induced
degradation, in which the initial conversion
efficiency of 10% for instance is degraded down toless than 8% by prolonged light irradiation. To
overcome this problem, although it is essentially
important to control the photo-induced degradation
through the control of network structure responsible
for this phenomenon in a-Si:H during film growth,tandem-type solar-cell structure has been proposed
which consists of thin a-Si:H top cell andmicrocrystalline silicon (µc-Si:H) bottom cellshowing no photo-induced degradation.

µc-Si:Ha-Si:H

metalTCO
ppiinnFig. 1. Band diagram of tandem-type thin-film
silicon solar cell and materials used in the cell.
Thin a-Si:H solar cell less than 0.1 microns does
not show any photo-induced degradation due tolarge internal built-in field in p-i-n solar-cellstructure by which recombination event ofphoto-excited electrons and holes in active i layer,
being considered as the trigger action forphoto-induced degradation, is avoided by efficientcarrier correction into p-side and n-side electrodes.
Transmitted light (red and infrared light) throughthin a-Si:H top cell is well absorbed by the presenceof µc-Si:H bottom cell whose optical band gap isnarrower than a-Si:H.
More than 18% in conversion efficiency istheoretically predicted in tandem-type thin film
silicon solar cells. However there are many
material issues to be solved for actualizing low-costand high-efficiency thin film silicon solar cells.
3. Material issues to be solved
We have to understand the growth process ofa-Si:H as well as µc-Si:H from SiH4-/H2 reactive
glow-discharge plasmas to improve optoelectronic
properties of those materials for achieving highconversion efficiency and to increase their growth
rates for actualizing low-fabrication cost. These
are the materials issues for low-cost /high-efficiency tandem-type thin film silicon solarcells

SiH3

SiH4

SiH3Fig. 2. Schematic sketch of surface-reaction processof a-Si:H growth by SiH3 radicals.
Figure 2 shows schematic concept of surface
reaction of a-Si:H sketched on the basis ofexperimental results obtained using a variety ofsurface-reaction-diagnostic techniques as well as
gas-phase-reaction-diagnostic techniques applied tothe growth process of a-Si:H and µc-Si:H from
SiH4-/H2-reactive plasmas.
Dominant film
precursor is assigned to be SiH3 radical for botha-Si:H and µc-Si:H growth, SiH3 reaching thefilm-growing surface starts diffusing on the surface
and abstract surface-covering bonded H to create areactive site, and another SiH3 diffusing on thesurface finds the site to form Si-Si bond.
Performance of a-Si:H and µc-Si:H solar cells is
mainly governed by the number density ofcarrier-recombination centers (dangling bonddensity in both cases) incorporated in active i layer,
therefore, relevant surface-reaction process fordetermining the number density of dangling bondsin the resulting film should be identified. In thisaspect, we have to keep in our mind the basic
concept of thin film growth, i. e., the microscopic
structure in the resulting material is determined by
the top-surface and sub-surface structures during
film growth, because the surface (or subsurface) at
one instant becomes a part of bulk at the next
instant by a successive film growth.
Many knowledge about the growth of a-Si:H andµc-Si:H have been accumulated in this decade, anddangling-bond density, for instance, has started tobe controlled.
Now we have to start challenging to control thephoto-induced degradation in a-Si:H.
TheRelationship between the degree of photo-induced
degradation in a-Si:H based solar cells and thenumber density of Si-H2 bonds incorporated ina-Si:H, and the relationship between the number
density of Si-H2 bonds in the resulting a-Si:H andthe contribution ratio of higher silane-related
reactive species (HSRS) such as Si4H9 during film
growth, and furthermore the relationship between
the contribution ratio of HSRS and the deposition
parameters such as substrate temperature andplasma parameters (electron temperature andelectron density) have recently been suggested.
Therefore, control of the plasma parameter is thekey point to improve the photo-induced degradation
properties in a-Si:H, and high-efficiency
tandem-type thin film silicon solar cells will bepractical when the photo-induced degradation in
a-Si:H is completely controlled, since two third of
total electricity generation is governed by a-Si:H
top cell in the tandem-type solar-cell structure.
Finally it should be noted here that large area
uniform growth of high quality materials at highgrowth rate is also crucial key issue for actual
industrialization of low-cost / high-efficiency thinfilm silicon solar cells, indicating a great
importance of large-area-uniform high-density
plasma production.